In hydropneumatic suspension systems a typical arrangement comprises a piston connected to a wheel or other ground engaging part such as a caterpillar track. The piston extends into a liquid chamber and forces the liquid from that chamber through a damping restrictor into a second chamber. A floating piston or flexible membrane separates the second chamber from a closed gas chamber so that liquid entering the second chamber compresses the gas to provide the spring in the suspension. In alternative arrangements a dashpot replaces the restricted passage between liquid chambers.
There are various limitations to such a system. One problem, which has been addressed, is the change in height with loading due to compression of the gas, and systems have been proposed to retain constant height by varying the mass of gas in the chamber. However, existing proposals require either a source of compressed gas and a venting means, the gas chamber then no longer being closed, such as described in U.S. Pat. No. 4,408,773, or a two-part gas chamber with connecting valve and a hydraulic pumping arrangement to cause transfer of gas between the two chambers effectively to change the mass of gas in the part of the chamber forming the spring. This latter arrangement is shown in UK specification 1602291.
These two specifications concern retaining constant height of chassis. On the other hand there are also instances when it is desired to adjust chassis height, irrespective of load, for example to enable vehicles to travel over different types of terrain, to vary the utilisation mode of the vehicle or to suit the speed of travel. Height adjustments such as raising the height of the chassis require significant energy input to raise the mass of the vehicle. Various `active` suspension systems have been designed in which this energy is provided through hydraulic, pneumatic or electrical servo mechanisms.
Such active suspension systems involving an external source of energy draw on the energy resources of the vehicle, thus leading to reduced fuel efficiency, extra weight and design complexity.
There are proposals for adjusting ride height that harness the very considerable energy generated within the suspension system from ground surface impacts and wheel patter. This energy, when not used, is normally dispersed as heat, for example in the damping element ,of the suspension system. However, the systems using this ground generated energy have been centred on the hydraulic part of hydropneumatic systems and still suffer from design complexity.
UK specification 1128092 shows an arrangement in which valves in a ported piston separating two hydraulic chambers are arranged to permit fluid flow from one chamber to another when the piston oscillates as a result of ground impact. A staggered porting arrangement for the valves provides differential transfer between the chambers when the piston oscillates about a position displaced from a predetermined mean, such as occurs when load is increased. As a result of the differential transfer the piston and the chassis height are restored to the predetermined mean. Such an arrangement provides automatic ride height adjustment, utilising ground impact energy, but does not provide selectable, variable ride height during vehicle motion.